Custody was the primary motive but these crime scenes are full of overkill, which also means big anger. Think of any big event in your own life--we don't get married, for example, just because we're in love. There's also the desire for kids, for regular companionship, for passion, for financial advantage. In a criminal trial, it makes sense to keep the motive clear and simple, something the jury can understand, but what was going on in the minds of these depraved people is probably complex and darker than any of us can imagine. On the simple side, it's likely that that Billy knew they would find some money. There might have been various kinds of jealousy or even rage that a young girl like Hanna couldn't be intimidated. But I don't discount that the planning and execution of these crimes was pleasurable to them. I mean that. I think they liked the power and control and secrecy and the sense that they got away with playing god with the lives of 8 people. Hence the tattoos. Just my opinion.